Honeybees sterilise their hives with antimicrobial resin, scientists have discovered.

In doing so, they give the whole colony a form of "social immunity", which lessens the need for each individual bee to have a strong immune system.

Although honeybee resin is known to kill a range of pathogens, this is the first time that bees themselves have been shown to utilise its properties.

The team published details of their discovery in the journal Evolution.

Honeybees in the wild nest in tree cavities.

When founding a new colony, they line the entire nest interior with a thin layer of resins that they mix with wax. This mixture is known as propolis.

They also use propolis to smooth surfaces in the hive, close holes or cracks in the nest, reduce the size of the entrances to keep out intruders, and to embalm intruders that they've killed in the hive that are too big to remove.

A number of studies have shown that propolis has a range of antimicrobial properties, but mostly in relation to human health. For example, numerous publications cite its effectiveness against viruses, bacteria and even cancer cells.

That is how Mike Simone, a PhD student from the University of Minnesota in St Paul, US, and his supervisor Professor Marla Spivak became interested.

Spivak and her colleagues had tested the effectiveness of honeybee propolis against the HIV-1 virus. They then progressed to see how it impacted bee pathogens, such as American foulbrood.

"This led us to wonder what other things propolis might be doing for the bees," said Simone.

In experiments funded by the US National Science Foundation, Simone's team painted the inside walls of hives with an extract of propolis collected from Brazil or Minnesota. This inside layer mimicked how propolis or resins would be distributed in a feral colony nesting in a tree cavity.

They then created colonies of honeybees and housed either in hives enriched with resin, or hives without the resin layer - to act as a control.

After one week of exposure they collected bees that had been born in each colony.

Genetic tests on these 7-day-old bees showed that those growing in the resin-rich colonies had less active immune systems.

"The resins likely inhibited bacterial growth. Therefore the bees did not have to activate their immune systems as much," said Simone.

"Our finding that propolis in the nest allows bees to invest less in their immune systems after such a short exposure was surprising. Resins in the hive have been thought of as a potential benefit to a honey bee colony, but this has never been tested directly."

Using resins to help sterilise the colony can be thought of as a type of "social immunity" said the researchers.

And it may partly explain why bees and other social insects, such as ants, collect resins to build their nests in the first place.

"Honeybees can use wax, which they produce themselves, to do all the things that they use resin for in the nest. So it is interesting to think about why they might go and collect resins," said Simone.

"Especially since resins, being sticky, are hard to manipulate and take a lot of energy for individual bees to gather in very small quantities."

There is also some evidence that some mammals and birds coat themselves in naturally-occurring plant resin in a bid to reduce infestations with parasites.
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